The present invention relates to projection displays and more specifically to reducing the moirxc3xa9 effect associated with rear-projection displays based on pixelated technologies, such as DMD, LCD, LCOS, etc.
The typical screen technology in rear projection displays utilizes a special dark-stripe structure to improve the ambient light rejection, which effectively provides a higher contrast display. This dark-stripe structure is simply an array of blackened vertical stripes, separated by regions allowing light to pass through. These regions allowing light to pass through, whether they may be transparent, translucent, diffuse, or another type of structure, will be referred to herein as clear stripes or simply stripes or structures allowing light to pass. For CRT based displays these screens work fine. However for pixelated (digital) displays, which utilize digital spatial light modulators (SLM) such as the micromirror device (DMD) or LCD technology, the current dark-stripe structure can interfere optically with the pixelated structure of the SLMs, causing interference fringes known as moirxc3xa9 patterns.
FIGS. 1a and 1b are top and front drawings, respectively, of a typical dark-stripe, or dark-stripe rear-projection screen. The backside of the screen, where the projected image enters, consists of a layer of small lenticular lens elements 10. The dark-stripe structure is fabricated on the opposite surface (from the lens elements) of the lenticular layer and consists of vertical black stripes 11 separated by transparent (clear) stripes 12. Next, a diffusion layer 13 is put on top of the dark-stripe layer to diffuse the light 15, coming through the transparent stripes 12, across the entire screen 150-154. Finally a hard coating layer 14 is applied on the outside surface of the screen for protection purposes.
In operation, the black stripes 11 tend to make the screen look dark to the viewer while still letting light pass through it. This provides adequate picture contrast for viewing in a room with ordinary lighting conditions (although not intended for use in direct sunlight).
In these display screens, the lenticular lens elements 10 are optimized to direct most of the available light to a viewing spot directly in front of the screen, where a typical viewer is likely to be located. As the viewer moves away from this central viewing point, either vertically or horizontally, the brightness will gradually decrease.
FIG. 2a is a Fourier transform of a continuous-time signal and FIGS. 2a and 2b are Fourier transforms of discrete-time signals obtained by periodic sampling this continuous signal, which illustrate what causes the moirxc3xa9 fringes in digital displays. In FIG. 2b the sampling period for the screen is large (low sampling rate), so that the periodic repetitions of the continuous-time transform (FIG. 2a) overlap. In this case, the upper frequencies in Xa(jxcexa9) (FIG. 2a) get reflected into the lower frequencies in X(ejxcfx89) (FIG. 2b) in the overlapped areas. This phenomenon, where in effect the high frequency component in the continuous time signal takes on the identity of a lower frequency, is called aliasing and causes moirxc3xa9 fringes to occur. On the other hand, in FIG. 2c the sampling period for the screen is small enough (high sampling rate) so that the periodic repetitions of the continuous time transform do not overlap and therefore moirxc3xa9 fringes do not occur.
FIG. 3 is an example of the moirxc3xa9 effect 32. This illustration involves overlaying one pixelated pattern 31 over a second pixelated pattern 30 and slightly rotating the patterns relative to each other to establish the overlapping conditions discussed in FIG. 2b. 
The pitch (spacing between lines) of dark-stripe screens is continuously getting smaller as screen technology advances, but so are the display pixels, so that moirxc3xa9 effects will continue to be a problem. What is needed is a method to provide a step-function improvement to overcome this problem. The disclosed invention accomplishes this by rotating the dark-stripe structure relative to the displayed pixels.
This invention discloses a method for reducing the moirxc3xa9 effect in rear-projection displays by rotating the dark-stripe structure in the screen 45xc2x115 degrees relative to the vertical axis of the display. By rotating the black stripes relative to the display pixels, the spatial frequency at which the moirxc3xa9 effect sets in can be improved by 15% to 41%.
To prevent the moirxc3xa9 phenomenon from occurring, the Nyquist rate of the screen structure must be less than one-half the pixel rate (as imaged on the screen); that is, the projected pixel pitch is greater than twice the screen structure pitch. The method of this invention extends the ratio of the projected pixel pitch to screen structure pitch by a factor of up to 1.41.
The lenticular lens elements on the input surface of the screen can be shaped to match the angle of the opening between dark-stripes to maintain a high brightness level in the center of the screen that decreases in the normal sense as the viewer moves in either the vertical or horizontal directions, if desired.
This method extends the use of existing lower-cost dark-stripe screens to provide high performance displays with reduced moirxc3xa9 interference and as screen technology advances with finer dark-stripe pitch, will continue to provide between 15% and 41% improvement over conventional vertical stripe screens.